1. Field of the Invention
The present invention relates to an overcurrent trip unit for an electrical circuit interrupting device, such as metal clad switchgear, molded case circuit breakers and the like, for protecting electrical conductors from damage due to excessive electrical currents and, more particularly, to a microprocessor based overcurrent trip unit with adjustable tripping characteristics that is adapted to continuously monitor the electrical current flowing through the electrical circuit interrupting device and initiate a trip as a function of a selectable tripping characteristic even during conditions when the current transformers driving the overcurrent trip unit are saturated wherein the overall reliability of the electrical distribution system is improved by allowing the time-current characteristic and in particular the long time delay and short time delay portions of the protection curve to be adjusted over a relatively wide range without overlap and further allows the time-current characteristic itself to be selected from a plurality of programmed functions (e.g., FLAT, It, I.sup.2 t, I.sup.4 t) to allow the overall coordination of the electrical distribution system to be improved in order to provide selective isolation of excessive electrical currents in an electrical distribution system, thus eliminating unnecessary circuit interruptions in the electrical system.
2. Description of the Prior Art
Various overcurrent devices are known in the art for protecting electrical conductors in an electrical distribution system from damage due to an excessive electrical current. Such overcurrent devices are typically characterized by their time-current characteristics or protection curve. Such protection curves are normally utilized to limit the temperature rise of an electrical conductor due to an excessive electrical current in order to prevent damage. For example, the temperature rise of the electrical conductors during certain excessive current conditions can be approximated by the product of the square of the electrical current and the time period that such electrical current is applied to the electrical conductors (e.g., I.sup.2 t). Thus, for an electrical motor rated for a predetermined temperature rise, for example, 55.degree. C., such overcurrent devices are used to limit the temperature rise of the electrical conductors within the motor to the rated temperature rise.
In order to facilitate selection of an overcurrent device with a suitable characteristic for use with an electrical motor, motor operating curves (for example, as shown in FIG. 2) are normally provided by a motor manufacturer. Such motor operating curves graphically illustrate the normal time and current characteristics of a particular electrical motor at its rated temperature rise. Accordingly, in order to protect the motor from damage and at the same time prevent spurious tripping of the motor during start-up, it is necessary to "coordinate" the motor operating curve with the time-current characteristics of an electrical overcurrent device utilized on the electrical circuit breaker feeding the motor.
It is also known to coordinate the overcurrent device provided on the electrical circuit breaker feeding circuits and loads protected by various overcurrent devices utilized in the electrical distribution system in order to prevent unnecessary tripping of such circuit breakers. Thus, the time-current characteristics for all of the various overcurrent devices in the electrical distribution system are coordinated to provide for "selective" tripping. Selective tripping refers to tripping of only those portions of an electrical distribution system necessary to isolate an excessive electrical current. Selective tripping provides for several advantages in an electrical distribution system.
First, selective tripping greatly improves the reliability of the electrical distribution system. For example, various electrical interrupting devices, for example, motor control centers, unit substations and the like, include a plurality of circuit breakers and the like for providing electrical power to various electrical loads. By utilizing selective tripping, a fault at or adjacent one of the electrical loads would result in only that load being isolated from the electrical distribution system. The balance of the electrical loads fed from the motor control center or the like would be undisturbed. As such, the reliability of the electrical distribution system is greatly improved.
Second, selective tripping facilitates the maintenance cost for locating and repairing the source of an excessive electrical current. More specifically, by utilizing selective tripping, only the circuit breaker or other protective device immediately upstream of the source of the excessive electrical current is tripped. Accordingly, the source of the excessive overcurrent can generally be located relatively quickly thereby decreasing the maintenance time and also decreasing the down time for the electrical load that was tripped. Moreover, such selective tripping also prevents unnecessary tripping of interrupting devices, such as fuses, which would require replacement thereby decreasing the maintenance cost and down time of the system.
Selective tripping further optimizes the cycle life of the circuit breakers in the electrical distribution system. More specifically, the various molded case circuit breakers and metal clad switchgear breakers within an electrical distribution system are generally adapted to operate a predetermined number of times before they either need to be replaced or serviced. This predetermined number is known as the cycle life. By preventing unnecessary operations of the various electric circuit breakers within the distribution system, the cycle life of the various breakers is thus improved.
Ideally, all of the various overcurrent devices provided in an electrical distribution system are coordinated to provide good electrical protection and to provide for selective tripping. However, in actuality, perfect coordination is not always attainable for several reasons. One reason relates to the particular time-current characteristics of the overcurrent devices. For example, fuses are known to be used for overcurrent devices in electrical distribution systems. The time-current characteristics of such fuses are significantly different from the time-current characteristics of various other overcurrent protection devices. Accordingly, it can be difficult to achieve good coordination of a fuse with other overcurrent devices over an entire range of anticipated overcurrents. Thus, in such applications, the coordinated protection may be less than ideal.
Another problem with known overcurrent devices relates to the adjustment ranges provided in such devices. More specifically, some known overcurrent devices are known to have rather limited adjustment ranges, for example, in the long time delay and short time delay portions of the protection curve, in order to prevent overlap which could degrade the selectivity of the system. Due to such limited adjustment ranges of such known overcurrent devices, the coordination of such overcurrent devices is relatively limited.
Another problem with known overcurrent devices, relates to their response to a relatively large overcurrent, such as a short circuit. More specifically, current transformers (CT) are known to be used to sense the electrical current flowing in an electrical interrupting device. The secondary windings of such CT's are then applied to an overcurrent device. During relatively large overcurrent conditions, such as a short circuit, the CT's often become saturated, thus resulting in a distorted current waveform on the secondary winding. Accordingly, during such a condition when the current transformers are saturated the overcurrent device, driven by such saturated current transformers, may not properly respond to such a distorted waveform during a short circuit condition in time to prevent damage.